This invention relates generally to computed tomography systems, and, more specifically, to flexible calibration phantoms which are scanned simultaneously with a portion of a body in connection with analysis of patients utilizing such systems.
In a common method of computed tomography, a patient is supported for translational movement along a longitudinal axis which coincides with the center of rotation of a gantry having an X-ray source on one side of the center of rotation and an X-ray detector on the other side. A pencil-like or fan-shaped X-ray beam is projected through the patient as the gantry rotates so that the detector may develop signals indicative of X-ray transmission characteristics along a plurality of paths through the patient. Analog signals representative of X-ray attenuation by all of the volume elements in a layer of the patient at various rotational angles are then converted to digital signals which are used by a computer to produce signals that may thereafter be used to produce a reconstructed image of the layer. The reconstructed image of the radiation attenuation coefficients may be displayed in gray tones on the screen of a visual display or on developed film.
One of the primary problems associated with computed tomography systems is proper calibration. The computer tomography system is quite complex and subject to occasional interference and signal drift which may result in a slight shift in performance. Another problem associated with system calibration is due to a phenomenon referred to as "beam hardening". The radiation attenuation coefficient is a property of every material and expresses the radiation absorption properties of the material at a specific X-ray energy.
The attenuation coefficients and corresponding gray tones produced by the computed tomography system are relative and must be calibrated to known values. Some systems are calibrated on a regular basis by scanning a set of known reference samples mounted in what is referred to as a calibration phantom. Prior art phantoms are available in a number of variations, some being plastic replicas of the human body, or specific portions thereof, while others consist of actual human bones cast in plastic. Some phantoms include a set of reference samples having known attenuation coefficients surrounded by a water medium housed within a plastic vessel.
Phantoms can also be used to correlate images from calibrated machines to analyze and interpret the images. In order to facilitate use of a phantom simultaneously with the analysis of a patient, a tabletop has been designed which includes a sample of reference material positioned to appear in each computed tomograph image. Such a computer tomography table containing calibration and correlation samples is shown in U.S. Pat. No. 4,233,507, the contents of which are incorporated herein.
In some instances it is desirable to place the phantom on or wrap it about a body part to be examined. Prior phantoms used simultaneously with the analysis of the patient, however, generally include a rigid encasement which holds a plurality of fluid-containing tubes, forming a tabletop upon which the patient lies. These rigid phantoms are considered unacceptable for regular use outside of the tabletop. Further, prior phantoms incorporating fluid reference samples often have air bubbles within the samples which can interfere with proper system calibration.
Accordingly, there has been a need for a novel calibration phantom which may be easily and comfortably placed directly on a body part to be examined, which also has sufficient flexibility to conform to normal body contours and thus eliminate air gaps between the body and the phantom. Additionally, there exists a need for a calibration phantom which utilizes soft and flexible samples of reference materials for reliable calibration of systems simultaneously with analysis of the patient. Moreover, a calibration phantom is needed which is relatively inexpensive to produce, durable, and capable of use in most existing computed tomography systems. The present invention fulfills these needs and provides other related advantages.